In an optical communication system, the limit of a transmission speed (bit rate of data) or total data transmission capacity (transmission speed per channel×number of channels), and a possible transmission distance depend on the optical S/N ratio (optical signal-to-noise ratio) and the waveform distortion and phase distortion of an optical signal. The waveform distortion and the phase distortion of an optical signal mainly depend on the chromatic dispersion (including higher-order dispersion) of transmission line optical fiber, a nonlinear optical effect, etc. In addition, the optical S/N ratio depends on the noise of amplified spontaneous emission (ASE) generated in an optical amplifier for compensating for a loss of optical fiber and the noise characteristic of a transmitter or a receiver.
There are the following techniques of compensating for the waveform distortion of an optical signal by chromatic dispersion.    (1) A transmission line alternately having normal dispersion fiber and anormalous dispersion fiber.    (2) A chromatic dispersion compensator such as a dispersion compensation fiber, etc.    (3) A configuration of performing electric signal processing after converting a received optical signal into an electric signal.
Up to now, there has been the development of an optical fiber transmission system for transmitting 10 Gbps data while compensating for a transmission loss using an optical amplifier. In addition, the development of a higher speed long distance data transmission (for example, 40 Gbps, 160 Gbps) and the development of a method for providing system margin capable of expanding for a photonic network have moved forward.
However, even the high-precision dispersion compensation and a high-quality optical amplifier are combined, waveform distortion remains and there occurs serious degradation of an optical S/N ratio due to ASE noise generated by the optical amplifier. Therefore, a practical transmission distance is limited. As a result, to realize a long-distance optical fiber transmission of a high-speed signal, there is a demand to realize an optical signal recovery device equipped with the technology of shaping a distorted optical waveform, correcting phase distortion, and suppressing accumulated ASE noise, phase noise, etc.
Well known as a related technique is an optical switch provided with a polarization controller, a nonlinear optical medium, and a polarizer. A polarization controller controls the direction of polarization of optical signal. The optical signal, the polarization of which is controlled by the polarization controller, is input to the nonlinear optical medium. The polarizer is provided at the output side of the nonlinear optical medium, and has a polarization axis orthogonal to the direction of polarization of the optical signal output from the nonlinear optical medium. The optical signal is parametrically amplified by a control light pulse in the direction of polarization of the control light pulse in the nonlinear optical medium. Thus, the optical signal of overlapping with the control light pulse in the nonlinear optical medium passes through the polarizer.
Known as technology of shaping the waveform of an optical signal is an optical waveform shaping device having first and second power controllers and a nonlinear optical medium. The first power controller controls the power of optical signal. The second power controller controls the power of the pumping light having a wavelength different from the wavelength of the optical signal. The power-controlled optical signal from the first power controller and the power-controlled pumping light from the second power controller are input to the nonlinear optical medium. Then, the first power controller controls the power of the optical signal so that the gain by the pumping light is saturated in the nonlinear optical medium. Thus, the function of an optical limiter is realized, and an optical waveform is shaped.
These techniques are disclosed by, for example, Japanese Laid-open Patent Publication No. 2006-184851 and Japanese Laid-open Patent Publication No. 2007-264319.
In the prior art technology, a system has mainly been supposed to have substantially constant input power of optical signal, or transmit data of with a single wavelength. Therefore, in the prior art technology, when the input power of optical signal fluctuates, the waveform of the optical signal is not completely shaped. Additionally, an optical signal processing device for collectively processing the waveforms of a plurality of optical signals transmitted in wavelength division multiplexing (WDM) light has not been realized. In the WDM system, when the number of channels to be multiplexed changes, the power of the WDM light changes accordingly.